Control of Ground‐Water Salinity, Orange County, California

ABSTRACT The Orange County Water District in Southern California imports approximately 75 percent of its supply from aqueducts drawing waters from the Colorado River and Northern California. Twenty‐five percent comes from natural flow in the Santa Ana River and local precipitation. Only about one‐half of the aqueduct water is treated and distributed in pipelines. The other one‐half is applied to infiltration basins and ponds for ground‐water recharge. This allows communities within the District to draw ground water as their major supply, which is supplemented by aqueduct water purchased after treatment by the Metropolitan Water District of Southern California. The economic advantages of ground‐water recharge, as opposed to direct use of surface supplies, include eliminating water treatment, minimizing the size of pipe networks to distribute potable supplies, and reducing the requirement for storage reservoirs. However, using the basin aquifers as a reservoir to balance cyclic supply and demand creates two possible problems: excessive pumping lowers the fresh‐water table below sea level, and infiltration can result in a gradual increase in the level of dissolved salts in the ground water. Sea‐water intrusion has been greatly reduced by injection wells that provide a hydraulic barrier between the salt water and fresh inland water. Salt accumulation in the basin is a more serious long‐term problem since the salinity in the Colorado River water is increasing. Greater use of less salty Northern California water and reducing unwanted seepage of waste waters containing high dissolved solids are two of the remedial measures being implemented at present.